The invention is concerned with a process for the production of cellulose fibres having a reduced tendency to fibrillation.
As an alternative to the viscose process, in recent years there has been described a number of processes in which cellulose, without forming a derivative, is dissolved in an organic solvent, a combination of an organic solvent and an inorganic salt, or in aqueous saline solutions. Cellulose fibres made from such solutions have received by BISFA (The International Bureau for the Standardization of man made Fibres) the generic name Lyocell. As Lyocell, BISFA defines a cellulose fibre obtained by a spinning process from an organic solvent. By "organic solvent", BISFA understands a mixture of an organic chemical and water. "Solvent-spinning" is considered to mean dissolving and spinning without the forming of a derivative.
So far, however, only one process for the production of a cellulose fibre of the Lyocell type has achieved industrial-scale realization. In this process, a tertiary amine-oxide, particularly N-methylmorpholine-N-oxide (NMMO), is used as a solvent. Such a process is described for instance in U.S. Pat. No. 4,246,221 and provides fibres which present a high tensile strength, a high wet-modulus and a high loop strength.
However, the usefulness of plane fibre assemblies, for example fabrics, made from the above fibres, is significantly restricted by the pronounced tendency of such fibres to fibrillate when wet. Fibrillation means the breaking up of the wet fibre in longitudinal direction at mechanical stress in a wet condition, so that the fibre gets hairy, furry. A fabric made from these fibres and dyed significantly looses color intensity as it is washed several times. Additionally, light stripes are formed at the abrasion and crease edges. The reason may be that the fibres consist of fibrils which are arranged in the longitudinal direction of the fibre axis and that the crosslinking between these fibres is reduced.
WO 92/14871 describes a process for the production of a fibre having a reduced tendency to fibrillation. The reduced tendency to fibrillation is attained by providing all the baths with which the freshly spun fibre is contacted before the first drying with a maximum pH of 8.5.
WO 92/07124 also describes a process for the production of a fibre having a reduced tendency to fibrillation, according to which the freshly spun, i.e. not yet dried, fibre is treated with a cationic polymer. As such a polymer, a polymer with imidazole and azetidine groups is mentioned. Additionally, there may be carried out a treatment with an emulsifiable polymer, such as polyethylene or polyvinylacetate, or a crosslinking with glyoxal.
In a lecture given by S. Mortimer at the CELLUCON conference in 1993 in Lund, Sweden, it was mentioned that the tendency to fibrillation rises as drawing is increased.
It is the objective of the invention to provide a process which can be carried out in an easy way, allowing the production of cellulose fibres of the Lyocell type having a reduced tendency to fibrillation.
The process according to the invention for the production of cellulose fibres having a reduced tendency to fibrillation consists in that the freshly spun, not yet dried fibres are contacted with a textile agent, which carries at least two reactive groups, and are washed with an aqueous buffer, provided glyoxal is not used as textile agent.
As textile agents, especially dyes having two reactive groups have shown good results. According to the invention, however, textile agents which are colorless, i.e. do not absorb visible light, may also be used.
According to the invention, the use of textile agents which carry at least one vinylsulfone group as reactive groups is preferred.
A convenient embodiment of the process according to the invention is characterized in that the freshly spun fibres are contacted with the textile agent in an aqueous alkaline medium.
It was shown that the tendency to fibrillation is especially reduced when an alkali carbonate and an alkali hydroxide is chosen as the alkaline medium.
Another preferred embodiment of the process according to the invention consists in that the fibres contacted with the textile agent are heat-treated. The heat treatment drastically reduces the impregnation time.
EP-A -0 538 977, published on Apr. 28, 1993, describes heat treatment of cellulose fibres impregnated with a dye. It was shown, however, that heating of the fibres impregnated with the textile agent by means of hot air may reduce the impregnation time, but that this carries the risk of the fibres not being heated uniformly. Thus for example, the fibres on the outside of a fibre bundle to be dried may already be partly dried, while the fibres inside the bundle have not yet reached the required temperature. This has a negative effect on the quality of the fibres produced.
It has been found that this drawback occurring in a simple heat treatment can be overcome by irradiating the fibres with electromagnetic waves, particularly microwaves. When irradiated with microwaves, on the one hand the fibres are heated uniformly and on the other hand premature drying of the fibres can be avoided. Since irradiation with electromagnetic waves provides the possibility to weld the fibre bundle for example into a plastic wrapping and expose it in a welded condition to the electromagnetic field.
The above advantages are also given when the fibres, for example lying as plane fibre assembly on a conveyor, are moved through a narrow channel, in which they are exposed to electromagnetic waves. This channel can be designed in a way that only little air space above the fibres is left, thereby preventing a partial drying of the outer fibres of the bundle. Simultaneously, this way of fixation of the textile agent also offers the possibility of a large-scale production easy to put into practice.
Thus, the invention further refers to a process for the production of cellulose fibres having a reduced tendency to fibrillation, in which a solution of cellulose in a tertiary amine-oxide is spun into fibres and the freshly spun fibres are contacted with a textile agent which carries at least two reactive groups and are heat-treated, characterized in that the heat treatment is carried out by irradiation with electromagnetic waves.
Also according to this embodiment of the process according to the invention, the use of a textile agent which carries vinylsulfone groups as the reactive groups, is preferred, preferably being a dye. However, textile agents which are colorless, i.e. do not absorb visible light, can also be used.
Another preferred embodiment of the process according to the invention consists in carrying out the heat treatment by means of microwaves.